TWI518186B - High strength hot-rolled steel sheet and method for producing the same - Google Patents

Description

High-strength hot-rolled steel sheet and manufacturing method thereof

The present invention relates to a high-strength hot-rolled steel sheet excellent in burring formability and a method for producing the same. The high-strength hot-rolled steel sheet of the present invention is mainly used for automotive body components, such as structural parts such as a member of a vehicle body or a structural part such as a frame or a chassis component such as a suspension ( Chassis parts). However, it is not limited to these uses.

In recent years, high-strength steel sheets have been actively used in raw materials for automatic vehicle parts in order to achieve weight saving of an automatic body. The use of high-strength steel sheets as structural parts of automatic vehicles has been popularized. Further, in order to further reduce the weight of the automatic vehicle body, it is strongly desired to apply a high-strength steel sheet not only to the frame member but also to the chassis member in which the hot-rolled steel sheet is generally used.

Most of the automatic vehicle parts using steel sheets are formed into a predetermined shape by press forming or burring forming of the steel sheets. However, in general, with the high strength of the steel plate, the steel plate The processability will be reduced. Therefore, high-strength steel sheets for automatic vehicle parts are required to have both required strength and excellent workability. In particular, since automatic vehicle chassis parts and the like are formed by strict processing, it is necessary to achieve high strength and workability at the same time. In particular, the quality of the edge processing is determined in many cases whether the high-strength steel sheet can be applied to the part or affects the mass productivity.

Previously, in order to improve the workability of high-strength hot-rolled steel sheets, various microstructure control or strengthening methods have been effectively utilized. For example, the composite structure of ferrite iron and martensite which are excellent in ductility, the effective utilization of the bainite microstructure, and the precipitation strengthening of the ferrite iron structure. However, in the prior art, a high-strength hot-rolled steel sheet having sufficient workability, which is capable of being applied to a part formed by performing a strict burring process such as an automatic undercarriage part, cannot be obtained, and high-strength heat having excellent workability is desired. Rolled steel plate.

In order to cope with the above-mentioned expectation, Patent Document 1 proposes a hot-rolled steel sheet having a composition containing C: 0.01% or more and 0.20% or less, Si: 1.5% or less, and Al: 1.5% or less in mass%. Mn: 0.5% or more and 3.5% or less, P: 0.2% or less, S: 0.0005% or more and 0.009% or less, N: 0.009% or less, Mg: 0.0006% or more and 0.01% or less, and O: 0.005% or less, and Ti: one or two types of 0.01% or more and 0.20% or less and Nb: 0.01% or more and 0.10% or less, and is a structure of a tough iron phase main body. Further, according to the technique proposed in Patent Document 1, the steel sheet structure is a structure of a tough iron phase main body, and the Mg-based sulfide is effectively utilized to refine (Ti, Nb) N, thereby obtaining over 980 N/mm ^{2 .} The hole expandability and the high-strength hot-rolled steel sheet excellent in ductility.

Further, Patent Document 2 proposes a hot-rolled steel sheet having a composition containing C: 0.01% or more and 0.10% or less, Si: 2.0% or less, and Mn: 0.5% or more and 2.5% by mass%. In addition, V is preferably 0.01% or more and 0.30% or less, Nb is 0.01% or more and 0.30% or less, Ti is 0.01% or more and 0.30% or less, and Mo is 0.01% or more and 0.30% or less. Zr: one or more of 0.01% or more and 0.30% or less, and W: 0.01% or more and 0.30% or less, and a structure having a toughening iron fraction of 80% or more. Further, according to the technique proposed in Patent Document 2, the steel sheet structure is made into a tough iron main body, and the toughened iron is precipitated and strengthened by carbides such as V, Ti, and Nb, whereby stretchability can be obtained (stretch- Flangeability) High-strength hot-rolled steel sheet excellent in fatigue characteristics.

Further, Patent Document 3 proposes a high-tension hot-dip galvanized steel sheet having a hot-rolled steel sheet as a substrate, and the hot-rolled steel sheet has a composition of C: 0.07% or more and 0.13% or less by mass%, Si. : 0.3% or less, Mn: 0.5% or more and 2.0% or less, P: 0.025% or less, S: 0.005% or less, N: 0.0060% or less, Al: 0.06% or less, Ti: 0.10% or more and 0.14% or less, and V: 0.15% or more and 0.30% or less, and a structure in which fine carbides having a ferrite-grained iron phase main body and having an average particle diameter of less than 10 nm are dispersed and precipitated at a desired volume ratio. Further, according to the technique proposed in Patent Document 3, a high-tension hot-dip galvanized steel sheet having excellent workability with a tensile strength of 980 MPa or more can be obtained.

Prior technical literature

Patent literature

Patent Document 1: Japanese Patent Laid-Open Publication No. 2005-120437

Patent Document 2: Japanese Patent Laid-Open Publication No. 2009-84637

Patent Document 3: Japanese Patent Laid-Open No. 2011-225978

However, in the techniques proposed in Patent Document 1 and Patent Document 2, since the hot-rolled steel sheet structure mainly composed of a toughened iron phase is used, the ductility of the hot-rolled steel sheet is lowered. Therefore, it is not possible to obtain a high-strength hot-rolled steel sheet having sufficient flushing workability that can be applied to an automatic vehicle part. Further, since the technique proposed in Patent Document 1 uses a high-priced Mg, it cannot be applied as a technique suitable for mass-produced parts such as automatic vehicle parts.

On the other hand, in the technique proposed in Patent Document 3, it is assumed that fine carbides are dispersed in a steel sheet structure in a matrix mainly composed of a ferrite-grained iron phase. Therefore, according to the technique proposed in Patent Document 3, a high-strength steel sheet having relatively good edge workability is obtained. Further, as shown in the examples, the high-strength steel sheet exhibits excellent punching workability by performing a hole expanding test according to the method of the Japan Iron and Steel Alliance Code.

However, in the prior art, which is based on the technique proposed in Patent Document 3, the punching workability required for mass production of automatic vehicle parts has not been studied.

The edge processing property of the steel sheet was previously evaluated mainly by the method of performing the hole-expanding test according to the method of the Japan Iron and Steel Federation Standard. . However, in the hole expansion test, it can be said that it is difficult to faithfully reproduce the actual life. Stamping processing steps and reaming processing steps when mass production of automatic vehicle parts on the production line. Therefore, even in the case of a steel sheet having excellent punching workability in the experimental evaluation according to the above-described regulations, there is a problem that processing defects frequently occur when mass-produced automatic vehicle parts.

Especially in the case of mass production of parts, only the evaluation of workability in the laboratory is not sufficient, and the processability of the raw materials needs to be ensured after further consideration of variations in processing conditions in mass production. In the prior art, no research has been conducted on the above problems, and it is not always possible to obtain the required workability and the processability required for mass production of automatic vehicle parts, particularly the edge processing property (hereinafter sometimes referred to as mass production flushing). High-strength hot-rolled steel sheet with workability. For example, in the techniques proposed in Patent Documents 1 to 3, it is extremely difficult to manufacture a high-strength hot-rolled steel sheet that satisfies the strict mass production edge processing property required by an automatic vehicle manufacturer or a component manufacturer.

As described above, although a large amount of research has been conducted on hot-rolled steel sheets excellent in the edge-engaging property (flushing workability), the existing methods for effectively utilizing the toughened iron phase or the effective use of carbides for the ferrite iron have been studied. In the method of the main body organization, excellent mass production edge processing property cannot be achieved.

The present invention advantageously solves the problems raised by the above-mentioned prior art, and an object of the invention is to provide a high-strength hot-rolled steel sheet having a tensile strength (TS) of 900 MPa or more and excellent in punching workability, in particular, mass-produced edge workability, and its manufacture. method.

In addition, the "production margin processing" mentioned here is by the rush rate. The burring ratio was evaluated by performing a 60° conical punch after punching with a 50 mmφ punch (clearance of stamping: 30%). Measured by a hole-expanding test, and the above-described mass production edge processing property is evaluated by a lambda value obtained by a hole expansion test method according to a conventional hole expansion test method, for example, the Japan Iron and Steel Union Code. The edge processing property is different.

In order to solve the above problems, the inventors of the present invention first studied the evaluation method of the mass production edge processing property. Previously, the edge processing property was evaluated by the lambda value obtained by the hole expanding test method specified by, for example, the Japan Iron and Steel Federation specification. In this case, the punching punch has a diameter of 10 mmφ. However, the inventors of the present invention have found that the edge processing property of an actual part production site sometimes deviates from the lambda value evaluated in the laboratory based on the Japanese Iron and Steel Alliance specification. Further, after further research, it was found that the punch was evaluated by a novel hole expansion test using a 50 mmφ punch (punching gap: 30%) and a 60° conical punch for reaming. The processability has a good correlation with the mass production stamping property and the mass production edge processing property.

Then, the inventors of the present invention used the above-described novel hole expanding test to evaluate the mass production edge processing property, thereby performing various factors affecting the high strength and workability of the hot rolled steel sheet, particularly mass production edge processing property. Active research.

Specifically, in the hot-rolled steel sheet based on the ferrite-grained iron phase main structure having a high ductility, nitrides, sulfides, carbides, and composite precipitates (for example, carbonitrides) of the compounds, that is, In consideration of all the precipitates which can be precipitated in the hot-rolled steel sheet, the high-strength of the hot-rolled steel sheet and the improvement in the productivity of the production edge are improved. The law has repeatedly carried out active research.

As a result, it was found that the total amount of V contained in the hot-rolled steel sheet and the amount of Ti (Ti*) which contributes to the formation of carbides in Ti are optimized, and the grain size of the precipitated carbide in the hot-rolled steel sheet is improved. With a ratio of carbides of less than 9 nm, a hot-rolled steel sheet having a tensile strength of 900 MPa or more and having a strict mass production edge processing property required for an actual automatic vehicle part production line can be obtained. Moreover, it has been found that not only the carbides but also the overall size of precipitates (nitrides, sulfides, carbides, and composite precipitates of these compounds) which can be precipitated in the hot-rolled steel sheet are controlled, thereby mass-producing the edge processing. The sex is further improved.

Furthermore, the inventors of the present invention have studied a method of controlling precipitates (nitride, sulfide, carbide, and composite precipitates of these compounds) precipitated in a hot-rolled steel sheet to a desired size. That is, it is controlled to impart the required strength (tensile strength of 900 MPa or more) to the hot-rolled steel sheet and the size required for excellent mass production edge processing. As a result, it has been found that it is necessary to adjust the Mn content of the hot-rolled steel sheet and the respective contents of S, N, Ti, and V to an appropriate amount, and to optimize the cooling and coiling conditions after the hot rolling conditions or hot rolling.

The present invention has been completed based on the above findings, and the gist thereof is as follows.

[1] A high-strength hot-rolled steel sheet having a composition in which S, N, Ti, and V satisfy the following formula (1), and contains C: 0.06% or more and 0.13 by mass%. % or less, Si: less than 0.5%, Mn: more than 0.5% and 1.4% or less, P: 0.05% or less, S: 0.005% or less, N: 0.01% or less, Al: 0.1% or less, and Ti: 0.05% or more. 0.25% or less, V: more than 0.15% and 0.4% The remaining portion contains Fe and unavoidable impurities; and has a ferrite-grained iron fraction of more than 90%, precipitates a Ti-containing carbide, and more than 70% of the carbide has a particle diameter of less than 9 nm. Ti*+V≧0.35...(1)

In the formula (1), Ti*=Ti-N×(48/14)-S×(48/32), and S, N, Ti, and V are contents (% by mass) of each element.

[2] The high-strength hot-rolled steel sheet according to the above [1], wherein 50% by mass or more of Ti is precipitated as a precipitate containing Ti having a particle diameter of less than 20 nm.

[3] The high-strength hot-rolled steel sheet according to the above [1] or [2], which further comprises, in addition to the above composition, Nb: 0.002% or more and 0.1% or less by mass%.

[4] The high-strength hot-rolled steel sheet according to any one of the above [1] to [3], further comprising, in addition to the above composition, Cu: 0.005% or more and 0.2% or less by mass%, and Ni: 0.005% or more and 0.2% or less, Cr: 0.002% or more and 0.2% or less, Mo: 0.002% or more and 0.2% or less, and Sn: 0.005% or more and 0.2% or less.

[5] The high-strength hot-rolled steel sheet according to any one of the above [1] to [4], further comprising, in addition to the above composition, B: 0.0002% or more and 0.003% or less by mass%.

[6] The high-strength hot-rolled steel sheet according to any one of the above [1] to [5] wherein: In addition to the above-described composition, at least one of Ca: 0.0002% or more and 0.005% or less, and rare earth metal (Rare Earth Metals, REM): 0.0002% or more and 0.03% or less is contained in mass%.

[7] A method for producing a high-strength hot-rolled steel sheet, which is characterized in that the steel material having the composition according to any one of the above [1], [3] to [6] is heated to 1,100 ° C or higher. After the finish rolling temperature is (Ar ₃ +25° C.) or more and the total reduction ratio of the final two finish stands is 60% or less, the average rolling rate is 40° C./s or more and is cooled. Take the temperature: 520 ° C or more and 680 ° C or less to take up.

[8] A method for producing a high-strength hot-rolled steel sheet, which is characterized in that the steel material having the composition according to any one of the above [1], [3] to [6] is heated to 1,100 ° C or higher. After the finish rolling temperature is (Ar ₃ +25° C.) or more and the total reduction ratio of the final two of the finish rolling is 60% or less, the cooling is performed at an average cooling rate of 40° C./s or more to obtain a coiling temperature: The coiling is performed at 500 ° C or more and 640 ° C or less, and after the pickling, the annealing treatment is performed at a soaking temperature of 760 ° C or lower, and plating treatment is performed by immersing in a hot-dip galvanizing bath.

[9] The method for producing a high-strength hot-rolled steel sheet according to [8] above, wherein the alloying treatment is performed after the plating treatment is performed.

According to the present invention, a high-strength hot-rolled steel sheet having a tensile strength of 900 MPa or more and excellent punching workability capable of withstanding processing in mass production of automatic vehicle parts is obtained. Therefore, according to the present invention, the high-strength hot-rolled steel sheet can be applied to a member of a vehicle body of an automatic vehicle or a structural member such as a frame, and further applied to a chassis member such as a suspension. The present invention contributes greatly to the weight reduction of these parts.

Further, according to the present invention, since a hot-rolled steel sheet having a tensile strength of 900 MPa or more and excellent mass-producing workability is obtained, the use thereof is not limited to automatic vehicle parts, and a wider use of the high-strength hot-rolled steel sheet can be expanded. Thereby achieving an additional effect in the industry.

Hereinafter, the present invention will be specifically described.

The high-strength hot-rolled steel sheet according to the present invention has a composition in which S, N, Ti, and V satisfy the following formula (1), and contains C: 0.06% or more and 0.13% by mass%. Hereinafter, Si: less than 0.5%, Mn: more than 0.5% and 1.4% or less, P: 0.05% or less, S: 0.005% or less, N: 0.01% or less, Al: 0.1% or less, Ti: 0.05% or more and 0.25. % or less, and V: more than 0.15% and 0.4% or less, the remainder contains Fe and unavoidable impurities; and the fraction of the iron phase of the ferrite is more than 90%, carbide containing Ti is precipitated, and the carbide is precipitated More than 70% of the particles with a particle size of less than 9 nm, Ti*+V≧0.35...(1)

In the formula (1), Ti*=Ti-N×(48/14)-S×(48/32), and S, N, Ti, and V are contents (% by mass) of each element.

First, the reason for limiting the component composition of the hot-rolled steel sheet of the present invention will be described. In addition, the % which shows the following component composition shows the mass % unless it demonstrates especially.

C: 0.06% or more and 0.13% or less

C is an important element in forming an appropriate carbide in a hot-rolled steel sheet to ensure the necessary strength of the steel sheet. In order to obtain a desired tensile strength (900 MPa or more), it is necessary to set the C content to 0.06% or more. On the other hand, when the C content exceeds 0.13%, the workability of the hot-rolled steel sheet is lowered, and the required edge workability cannot be ensured. Therefore, the C content is set to 0.06% or more and 0.13% or less. It is preferably 0.07% or more and 0.12% or less.

Si: less than 0.5%

When the Si content is 0.5% or more, the surface properties of the hot-rolled steel sheet are remarkably lowered, which adversely affects fatigue characteristics, chemical treatment properties, corrosion resistance, and the like. Further, since Si increases the fermented iron metamorphic temperature, it adversely affects the formation of fine precipitates which are the objects of the present invention. Therefore, the Si content is set to be less than 0.5%. It is preferably 0.001% or more and less than 0.1%, more preferably 0.001% or more and less than 0.05%.

Mn: more than 0.5% and less than 1.4%

Mn is one of the most important elements in the invention. Mn, through the control of austenite-to-ferrite transformation temperatures, has a significant effect on the precipitation of the most important Ti-containing carbides in the present invention.

In the case of a hot rolled steel sheet containing Ti, the carbide containing Ti is mainly In the cooling and coiling process after the completion of the finish rolling in the hot-rolled steel sheet manufacturing step, the Worthite iron → fat iron is metamorphosed. Further, among the carbides precipitated in the hot-rolled steel sheet, the high strength of the hot-rolled steel sheet contributes to the fine carbide, and the coarse carbide does not contribute to the high strength and the processing of the hot-rolled steel sheet. Sexually adverse effects.

Here, when the Worthite-fertilizer iron metamorphic point reaches a high temperature, the Ti-containing carbide precipitates in a high temperature region, and thus the Ti-containing carbide is coarsened. Therefore, in terms of refining the carbide containing Ti, it is preferred to reduce the Wrestfield iron-fertilizer iron deformation point.

Mn is an element having an effect of lowering the deformation point of the Worthite iron-fertilizer iron. In the case where the Mn content is 0.5% or less, the Worstian iron-fertilizer iron deformation point is not sufficiently lowered. As a result, the carbide containing Ti is coarsened, and it is not possible to obtain a high-strength hot-rolled steel sheet excellent in mass production workability which is the object of the present invention. On the other hand, when the Mn content exceeds 1.4%, Mn segregation in the central portion of the thickness increases. This center segregation breaks the punched surface of the punched hole before the burring process, and thus causes deterioration in the productivity of the production edge. Therefore, the Mn content is set to be more than 0.5% and 1.4% or less. It is preferably more than 0.7% and 1.4% or less, more preferably more than 1.0% and 1.4% or less.

P: 0.05% or less

P may cause deterioration in workability of the hot-rolled steel sheet due to segregation or the like. Therefore, the P content is suppressed to 0.05% or less. It is preferably 0.001% or more and 0.03% or less. Among them, a galvanized treatment is performed on a hot-rolled steel sheet to form a galvanized steel sheet. In the case of the (galvanized steel sheet), the P content is preferably 0.005% or more, and more preferably 0.01% or more from the viewpoint of the plating property.

S: 0.005% or less

S forms a sulfide to lower the workability of the hot rolled steel sheet. Therefore, the S content is set to 0.005% or less. It is preferably 0.0001% or more and 0.003% or less, more preferably 0.0001% or more and 0.0015% or less.

N: 0.01% or less

If the N content exceeds 0.01%, it is excessive, and a large amount of nitride is formed in the step of producing the hot-rolled steel sheet, whereby the hot ductility is deteriorated, or the nitride is coarsened, and the edge workability of the hot-rolled steel sheet is remarkably deteriorated. Therefore, the N content is set to 0.01% or less. It is preferably 0.0001% or more and 0.006% or less, more preferably 0.0001% or more and 0.004% or less.

Al: 0.1% or less

Al is an important element as a deoxidizer for steel. However, if the content exceeds 0.1%, casting of steel becomes difficult, or a large amount of inclusions remain in the steel, resulting in deterioration of surface properties or workability of the hot-rolled steel sheet. Therefore, the Al content is set to 0.1% or less. It is preferably 0.001% or more and 0.06% or less.

Ti: 0.05% or more and 0.25% or less

Ti is one of the most important elements in the invention. Ti forms fine carbides and contributes to the strength improvement of the hot rolled steel sheet. In order to obtain the required strength of the hot-rolled steel sheet (tensile strength of 900 MPa or more), it is necessary to set the Ti content to 0.05% or more. On the other hand, if the Ti content exceeds 0.25%, coarse carbides tend to remain in the hot-rolled steel sheet, not only It is ineffective for strength improvement and can significantly deteriorate the workability, toughness and weldability of hot-rolled steel sheets. Therefore, the Ti content is set to be 0.05% or more and 0.25% or less. It is preferably 0.08% or more and 0.20% or less.

V: more than 0.15% and less than 0.4%

V is also one of the most important elements in the invention. V forms fine carbides to contribute to the strength improvement of the hot rolled steel sheet. In order to obtain the required strength of the hot-rolled steel sheet (tensile strength of 900 MPa or more), it is necessary to set the V content to more than 0.15%. On the other hand, if the V content exceeds 0.4%, an effect commensurate with the cost cannot be obtained. Therefore, the V content is set to be more than 0.15% and 0.4% or less. It is preferably more than 0.15% and 0.35% or less.

The hot-rolled steel sheet of the present invention contains S, N, Ti, and V so as to satisfy the following formula (1) within the above range. The formula (1) is a requirement for achieving high strength of the hot-rolled steel sheet and excellent mass production of the hot-rolled steel sheet, and is an extremely important index in the present invention. Further, in the formula (1), Ti*=Ti-N×(48/14)-S×(48/32), and S, N, Ti, and V are contents (%) of each element.

Ti*+V≧0.35...(1)

As described later, in the present invention, a predetermined amount of Ti and V as carbide formation elements are added to the steel material, and the carbide in the steel material is solid-solved during heating before hot rolling. These elements are mainly precipitated as carbides during coiling after hot rolling. However, added to the steel raw materials Ti, V Not all of the total amount contributes to the formation of carbides, and in particular, a part of Ti added to the steel raw material is easily consumed at the time of formation of a nitride or a sulfide. This is because in a temperature region higher than the coiling temperature, Ti is more likely to form nitrides or sulfides than carbides, so that in the production of hot-rolled steel sheets, Ti forms nitrides or sulfides before the coiling step. . Thus, the minimum amount of Ti that can contribute to carbide formation in Ti added to the steel raw material can be expressed by Ti* (=Ti-N×(48/14)−S×(48/32)).

Here, if Ti*+V is less than 0.35, the required hot-rolled steel sheet strength (tensile strength: 900 MPa or more) cannot be obtained. Further, when Ti*+V is less than 0.35, coarse nitride or sulfide is likely to be precipitated in the hot-rolled steel sheet, and excellent mass-produced edge workability cannot be obtained. Therefore, in the present invention, Ti*+V is set to 0.35 or more. Further, it is preferable to set Ti*+V to 0.355 or more. However, when Ti*+V exceeds 0.46, the strength of the hot-rolled steel sheet becomes too high, and the workability is deteriorated. Therefore, Ti*+V is preferably set to 0.46 or less.

The above is the basic component of the hot rolled steel sheet of the present invention. Further, the hot-rolled steel sheet according to the present invention may contain Nb: 0.002% or more and 0.1% or less, as needed.

Nb has an effect of refining crystal grains to improve the toughness of the hot-rolled steel sheet, and thus may be added as needed. In order to obtain the above effects, the Nb content is preferably made 0.002% or more. However, if the Nb content exceeds 0.1%, an effect commensurate with the cost cannot be obtained. Therefore, the Nb content is preferably 0.002% or more and 0.1% or less, more preferably 0.002% or more and 0.08% or less.

Moreover, the hot-rolled steel sheet of the present invention may also contain Cu: 0.005% as needed. Above 0.2% or less, Ni: 0.005% or more and 0.2% or less, Cr: 0.002% or more and 0.2% or less, Mo: 0.002% or more and 0.2% or less, and Sn: 0.005% or more and 0.2% or less. Kind.

Cu, Ni, and Sn are elements that contribute to the strength improvement of the hot-rolled steel sheet, and may be added as needed. In order to obtain the above effects, the Cu content is preferably 0.005% or more, the Ni content is 0.005% or more, and the Sn content is 0.005% or more. However, if the content of these elements exceeds 0.2%, there is a possibility that the surface layer is cracked during hot rolling when the hot rolled steel sheet is produced. Therefore, the Cu content is preferably 0.005% or more and 0.2% or less, more preferably 0.005% or more and 0.1% or less. Further, the Ni content is preferably 0.005% or more and 0.2% or less, more preferably 0.005% or more and 0.15% or less. Further, the Sn content is preferably 0.005% or more and 0.2% or less, more preferably 0.005% or more and 0.1% or less.

Both Cr and Mo are carbide-forming elements, which contribute to the improvement of the strength of the hot-rolled steel sheet, and may be added as needed. In order to obtain the above effects, the Cr content is preferably made 0.002% or more, and the Mo content is made 0.002% or more. However, if the content of these elements exceeds 0.2%, an effect commensurate with the cost cannot be obtained. Therefore, the Cr content is preferably 0.002% or more and 0.2% or less, and more preferably 0.002% or more and 0.1% or less. Further, the Mo content is preferably 0.002% or more and 0.2% or less, more preferably 0.002% or more and 0.1% or less.

Further, the hot-rolled steel sheet of the present invention may contain B: 0.0002% or more and 0.003% or less as needed.

B is an element that delays the deformation of the steel's Worthite-fertilizer iron by suppressing The Worthite iron-fertilizer iron metamorphosis lowers the precipitation temperature of the Ti-containing carbide, thereby contributing to the miniaturization of the carbide. In order to obtain the above effects, the B content is preferably made 0.0002% or more. On the other hand, when the B content exceeds 0.003%, the toughened iron metamorphic effect by B is enhanced, and it is difficult to set the hot-rolled steel sheet structure as the main phase of the ferrite-iron main phase of the present invention. Therefore, the B content is preferably 0.0002% or more and 0.003% or less, more preferably 0.0002% or more and 0.002% or less.

Further, the hot-rolled steel sheet according to the present invention may contain at least one of Ca: 0.0002% or more and 0.005% or less, and REM: 0.0002% or more and 0.03% or less, as needed.

Ca and REM are elements effective for controlling the morphology of inclusions in steel, and contribute to the improvement of workability of hot-rolled steel sheets. In order to obtain the above effects, the Ca content is preferably made 0.0002% or more, and the REM content is made 0.0002% or more. However, when the Ca content exceeds 0.005%, or when the REM content exceeds 0.03%, the inclusions in the steel increase and the workability of the hot-rolled steel sheet deteriorates. Therefore, the Ca content is preferably 0.0002% or more and 0.005% or less, more preferably 0.0002% or more and 0.003% or less. Further, the REM content is preferably 0.0002% or more and 0.03% or less, more preferably 0.0002% or more and 0.003% or less.

In the present invention, the components other than the above are Fe and unavoidable impurities. Examples of the unavoidable impurities include W, Co, Ta, Sb, zr, and O. The content of these impurities is allowed to be 0.1% or less.

Next, the reason for limiting the structure of the hot-rolled steel sheet of the present invention will be described.

In the hot-rolled steel sheet according to the present invention, the fraction of the ferrite-rich iron phase exceeds 90%. A precipitated carbide containing Ti and a structure in which 70% or more of the carbide has a particle diameter of less than 9 nm is precipitated. In addition, it is preferable that 50% by mass or more of Ti contained in the hot-rolled steel sheet is precipitated as a precipitate having a particle diameter of less than 20 nm.

Fraction of iron phase of fat grain: more than 90%

In order to improve the punching workability of the hot-rolled steel sheet, it is effective to set the hot-rolled steel sheet structure to a ferrite-rich iron phase excellent in ductility. In order to achieve the mass production edge processing property of the object of the present invention, it is necessary to set the ferrite iron fraction with respect to the entire structure of the hot-rolled steel sheet to be more than 90% in terms of area ratio. It is preferably more than 92% in terms of area ratio, more preferably more than 94% in area ratio. Further, from the viewpoint of the edge processing property, it is desirable that the shape of the ferrite particles is polygonal. Further, it is desirable that the particle size of the ferrite is extremely fine. Further, from the viewpoint of the edge processing property, it is preferable to set the hot-rolled steel sheet structure to a single-phase structure of the ferrite-iron. In addition, it is preferable that the iron content of the fertilizer is 98% or less in terms of area ratio, and more preferably 97% or less in area ratio, from the viewpoint of improvement in the pressability.

In the hot-rolled steel sheet according to the present invention, examples of the structure other than the ferrite-grain iron phase which may be contained include cementite, pearlite, toughened iron, 麻田散铁, and residual Worthite iron. (γ) (retained austenite) and the like. If the structures are excessively present in the steel sheet, the edge workability is lowered. However, it is acceptable if the total fraction of the structures is less than 10% in terms of the area ratio. Further, if these structures are present in an appropriate amount in the hot-rolled steel sheet, the pressability in the pre-stage of the burring process is improved, and the punching workability is further improved. Therefore, the total fraction of the tissues other than the ferrite iron phase is preferably 2% or more and less than 8% in terms of the area ratio, and more preferably set. The area ratio is 3% or more and less than 6%.

Ti-containing carbide

In the present invention, the carbide containing Ti is deposited in a hot-rolled steel sheet to impart a desired strength (tensile strength of 900 MPa or more) to the hot-rolled steel sheet. The Ti-containing carbide is mainly a carbide precipitated in the cooling and coiling step after completion of finish rolling in the hot-rolled steel sheet manufacturing step, accompanied by the transformation of the Worthite iron to the ferrite iron.

In order to maximize the effect of precipitation strengthening, the balance between strength and workability (mass production edge processing property) is optimized, and it is necessary to refine the Ti-containing carbide precipitated in the hot-rolled steel sheet. As a result of active research, the inventors of the present invention have found that in order to achieve desired characteristics, it is necessary to set a carbide having a number of Ti-containing carbides of 70% or more to a particle diameter of less than 9 nm. It is preferably 80% or more. Here, the "Ti-containing carbide" includes, in addition to Ti, one or more composite carbides of V, Nb, Cr, and Mo in addition to Ti.

Precipitate containing Ti

By controlling the size of the precipitate containing Ti, it is possible to further improve the mass production processability of the hot-rolled steel sheet.

As described above, in the case of a hot-rolled steel sheet containing a steel containing Ti as a material, in addition to the precipitation of carbides (Ti-containing carbides) which contribute to the high strength of the hot-rolled steel sheet, the nitride containing Ti, Carbonitrides, sulfides, and the like are also precipitated. Further, in the production of the hot-rolled steel sheet, the nitrides, carbonitrides, sulfides, and the like are precipitated first than the carbides containing Ti. Therefore, nitrides, carbonitrides, and sulfides containing Ti are precipitated in a temperature region higher than precipitation of carbides, and thus are easily coarsened and easily formed. Reduce the productivity of mass production.

As a result of active research, the present inventors have found that controlling the amount of precipitation and the particle diameter of these precipitates is extremely effective for improving the productivity of the mass production of the main object of the present invention. In order to achieve the above effects, it is preferred that 50% or more of Ti contained in the hot-rolled steel sheet be precipitated as a precipitate containing Ti having a particle diameter of less than 20 nm. It is more preferably 60% by mass or more and 85% by mass or less, and still more preferably 65% by mass or more and 80% by mass or less. The precipitate containing Ti having a particle diameter of less than 20 nm is mostly a carbide containing Ti, and a part thereof contains a nitride containing Ni, a carbonitride, or a sulfide.

Here, in addition to precipitates such as Ti carbide, Ti nitride, Ti sulfide, and Ti carbonitride, the precipitate containing Ti contains one of V, Nb, Cr, and Mo in addition to Ti. The above composite precipitates such as composite carbides, composite nitrides, composite sulfides, and composite carbonitrides.

In addition, when precipitates having a particle diameter of 20 nm or more are precipitated in the precipitate containing Ti, it is estimated that the amount of precipitation is appropriate, which contributes to the improvement of the punchability in the pre-stage of the edge processing, and further contributes The processing of the edge is improved.

Further, even if a plating layer is provided on the surface of the hot-rolled steel sheet of the present invention in order to impart corrosion resistance, the effects of the present invention described above are not impaired. The type of the plating layer provided on the surface of the hot-rolled steel sheet is not particularly limited, and may be any of galvanic electroplating and hot-dip plating. Further, examples of the hot-dip galvanization include hot-dip galvanization. Further, alloying treatment (alloying) may be performed after plating. Treatment) alloyed hot-dip galvanizing (galvannealed steel).

Next, a method of producing the hot-rolled steel sheet of the present invention will be described.

The present invention is characterized in that the steel material having the above-described composition is heated to 1,100 ° C or higher, and the hot rolling is performed at a finishing rolling temperature of (Ar ₃ + 25 ° C) or more and a total reduction ratio of two final rolls of the final rolling is 60% or less. Thereafter, the film was cooled at an average cooling rate of 40 ° C/s or more, and coiled at a coiling temperature of 520 ° C or more and 680 ° C or less.

In the present invention, the method of melting the steel material is not particularly limited, and for example, it is melted by a converter, an electric furnace, an induction furnace, or the like. Then, it is preferable to perform secondary smelting by using vacuum degassing equipment or the like. From the viewpoint of productivity or quality, it is preferred that the subsequent casting be carried out by a continuous casting process. Alternatively, it may be a method of utilizing block rolling. The cast slab (steel material) is a normal billet having a thickness of about 200 mm to 300 mm, and may be a thin billet having a thickness of about 30 mm. If it is a thin billet, rough rolling can be omitted. The cast slab can be directly subjected to hot direct rolling, or can be hot rolled after being heated by a heating furnace.

Heating temperature of steel raw materials: above 1100 °C

The steel raw material obtained as described above is subjected to hot rolling. In the present invention, it is important to heat the steel raw material (steel billet) before hot rolling to re-solidify the carbide in the steel raw material. When the heating temperature of the steel raw material is less than 1100 ° C, the carbide in the steel raw material is not dissolved again, and the cooling and coiling steps after the hot rolling is completed The desired fine carbides could not be obtained. Therefore, the heating temperature of the steel material is set to 1100 ° C or higher. It is preferably 1200 ° C or higher, more preferably 1240 ° C or higher.

However, if the heating temperature of the steel material becomes too high, the oxidation of the surface of the steel sheet is excessively promoted, and the surface quality is remarkably deteriorated, which also adversely affects the workability of the hot-rolled steel sheet. Therefore, the heating temperature of the steel material is preferably set to 1350 ° C or lower.

After the heating of the steel raw material, hot rolling including rough rolling and finish rolling is performed on the steel raw material. The rough rolling conditions are not particularly limited. Further, as described above, in the case where the steel material is a thin steel slab, the rough rolling may be omitted. In the finish rolling, the finish rolling temperature is set to (Ar ₃ + 25 ° C) or more, and the total reduction ratio of the final two seats of the finishing mill is set to 60% or less.

Finishing temperature: (Ar ₃ +25 ° C) or more

In the case where the finishing rolling temperature is less than (Ar ₃ + 25 ° C), the Worstian iron → fertilized iron in the cooling and coiling steps after the completion of the hot rolling becomes metamorphosis from the unrecrystallized austenite grain. The fat grain iron metamorphosis. In this case, the required fine carbides were not obtained, and the strength (tensile strength of 900 MPa or more) of the hot-rolled steel sheet which is the object of the present invention could not be attained. Therefore, the finish rolling temperature is set to (Ar ₃ + 25 ° C) or more. It is preferably (Ar ₃ + 40 ° C) or more. However, if the finish rolling temperature is too high, the crystal grains become coarse and adversely affect the punchability of the hot-rolled steel sheet. Therefore, the finish rolling temperature is preferably set to (Ar ₃ + 140 ° C) or less.

In addition, the Ar ₃ metamorphosis point mentioned here is a thermomechanical test (thermo-mechanical simulation test) at a cooling rate of 5 ° C/s to determine a thermal expansion curve (thermal expansion curve) Curve), and the metamorphic temperature determined based on the point of change.

The total reduction ratio of the final two blocks of finish rolling: 60% or less

In the case where the total reduction ratio of the final two of the finish rolling is more than 60%, the residual strain is increased to promote the metamorphosis of the ferrite from the unrecrystallized γ grains. Therefore, the total reduction ratio of the final two seats of the finishing mill is set to 60% or less. It is preferably 50% or less.

Average cooling rate: 40 ° C / s or more

When cooling is performed after the end of hot rolling, the average temperature of the fertilized iron is increased in the case where the average cooling rate is less than 40 ° C / s. As a result, carbides are precipitated in a high temperature region, and the desired fine carbides are not obtained, so that the strength (tensile strength of 900 MPa or more) of the hot-rolled steel sheet which is the object of the present invention cannot be attained. Therefore, the average cooling rate is set to 40 ° C / s or more. It is preferably set to 50 ° C / s or more. However, if the average cooling rate is too large, the desired ferrite iron structure may not be obtained, and therefore it is preferably 150 ° C / s or less.

In addition, the average cooling rate mentioned here is the average cooling rate between the finish rolling temperature and the coiling temperature.

In the present invention, cooling is performed at the above average cooling rate to lower the fermented iron metamorphic temperature to the vicinity of the coiling temperature, whereby the Ti-containing carbide is precipitated from immediately before the winding up to the initial stage of the winding step. Thereby, it is possible to prevent the carbide containing Ti from being precipitated and coarsened in a high temperature region, thereby obtaining a hot rolled steel sheet in which fine carbides are precipitated in the present invention.

Coiling temperature: 520 ° C or more and 680 ° C or less

As described above, in the present invention, the fine carbide containing Ti is mainly wound up, that is, Precipitate from the beginning to the beginning of the winding step. Therefore, in order to make the carbide containing Ti fine and precipitate a large amount, it is necessary to set the coiling temperature to a temperature region suitable for precipitation of carbide containing Ti. When the coiling temperature is less than 520 ° C or exceeds 680 ° C, the fine carbides contributing to the increase in strength of the steel are not sufficiently analyzed, and the desired strength of the hot-rolled steel sheet cannot be obtained. For the above reasons, the coiling temperature is set to 520 ° C or more and 680 ° C or less. It is preferably set to 550 ° C or more and 650 ° C or less.

In the present invention, the hot-rolled steel sheet after the coiling may be subjected to pickling and annealing treatment, and then subjected to a plating treatment immersed in a molten zinc bath. Further, after the plating treatment, the alloying treatment may be performed. In the case of performing the plating treatment, the coiling temperature is set to 500 ° C or more and 640 ° C or less, and the soaking temperature of the annealing treatment is set to 760 ° C or lower.

Coiling temperature: 500 ° C or more and 640 ° C or less

As the coiling temperature increases, an internal oxidation layer is easily formed in the hot rolled steel sheet. The internal oxide layer is a factor of poor plating, and in particular, if the coiling temperature exceeds 640 ° C, the plating quality cannot be ensured. On the other hand, from the viewpoint of suppressing plating failure, it is preferable to set the coiling temperature to be low. However, if the coiling temperature is less than 500 ° C, the precipitation amount of the carbide containing Ti cannot be sufficiently ensured, and the desired strength of the hot-rolled steel sheet cannot be obtained. Therefore, when the plating treatment is performed after the winding, the coiling temperature is set to 500 ° C or more and 640 ° C or less. It is preferably 520 ° C or more and 600 ° C or less.

Soaking temperature: below 760 °C

As described above, in the case where the plating treatment is performed, the coiling temperature is set. Since it is low, the fine carbide (Ti-containing carbide) which contributes to the high strength of the hot-rolled steel sheet is not fully analyzed at the time of winding. Therefore, in the present invention, fine carbides (Ti-containing carbides) are precipitated during the annealing treatment before the plating treatment, so that the hot-rolled steel sheets after the plating treatment have the required strength (tensile strength of 900 MPa or more). ). Here, when the soaking temperature of the annealing treatment exceeds 760 ° C, the precipitated carbide (carbide containing Ti) is coarsened, and the strength of the hot-rolled steel sheet is lowered. Therefore, the soaking temperature of the annealing treatment is set to 760 ° C or lower. It is preferably 740 ° C or lower. In addition, from the viewpoint of promoting precipitation of fine carbides (Ti-containing carbides), it is preferred to set the soaking temperature of the annealing treatment to 600 ° C or higher. Further, the holding time at the soaking temperature is preferably set to 10 s or more and 1000 s or less.

After the annealing treatment, the steel sheet was immersed in a hot-dip galvanizing bath to form a hot-dip galvanized layer on the surface of the steel sheet. It may also be alloyed after being immersed in a hot-dip galvanizing bath. The annealing treatment and the plating treatment are preferably carried out on a continuous hot-dip galvanizing line.

Further, the type of plating may be not only the above-described hot-dip galvanizing or alloying hot-dip plating, but also electrogalvanizing.

Further, the plating treatment conditions, the alloying treatment conditions, and other production conditions are not particularly limited, and for example, they can be carried out under normal conditions.

Example

The billet (No. A to No. P) having the composition shown in Table 1 and the Ar ₃ metamorphic point was heated to 1200 ° C to 1290 ° C, and a hot rolled steel sheet (No. 1) was produced under the hot rolling conditions shown in Table 2. ~No.22). The plate thickness is 1.2mm~3.2mm. Further, the Ar ₃ metamorphic point of Table 1 was determined by the method described above. Some hot-rolled steel sheets (No. 3, No. 4, No. 9, No. 10, No. 14, No. 16, No. 18 to No. 20) are pickled, and then passed through a hot-dip galvanizing line. This was subjected to an annealing treatment at the soaking temperature shown in Table 2, and further subjected to a hot-dip galvanizing treatment. Further, the hot-dip galvanizing treatment is a treatment in which the hot-rolled steel sheet after the annealing treatment is immersed in a galvanizing bath (0.1% by mass of Al-Zn) at 480 ° C, and the melting amount per side is 45 g/m ² . A hot-dip galvanizing layer is formed on both sides of the steel sheet. Further, a part of the hot-rolled steel sheets (No. 9, No. 10, No. 14, No. 16, No. 18 to No. 20) is subjected to a hot-dip galvanizing treatment, and then an alloying treatment is further performed. The alloying treatment temperature was set to 520 °C.

From the hot-rolled steel sheets (No. 1 to No. 22) obtained above, test pieces were taken, and microstructure observation, tensile test, and hole expansion test were performed. The tissue observation method and various test methods are as follows.

(i) Organizational observation

Fraction of iron phase

The hot-rolled steel sheet was subjected to a scanning electron microscope (SEM) test piece, and the thickness profile parallel to the rolling direction was polished, and then nitric etching was performed, and the thickness was 1/4. Position, SEM photographs were taken in 10 fields at a magnification of 3000 times, and the ferrite iron phase was separated from the ferrite iron by image analysis, and the fraction (area ratio) of each phase was determined.

Ti-containing carbide

Thin-film sample is produced from hot-rolled steel sheet (1/4 inch plate thickness), A 200,000-fold photograph was taken in 10 fields using a transmission electron microscope.

Based on the photograph taken, the total number of carbides containing Ti (N ₀ ) is determined, and the respective particle diameters of the Ti-containing carbides are taken as an equivalent circle diameter by image processing. The number (N ₁ ) of carbides having a particle diameter of less than 9 nm among the carbides containing Ti was determined. Using these values (N ₀ and N ₁ ), the ratio of the number of carbides of less than 9 nm to the total number of carbides (N ₁ /N ₀ ×100 (%)) was determined for the carbide containing Ti.

Precipitate containing Ti

The hot-rolled steel sheet was subjected to constant-current electrolysis using an AA-based electrolytic solution (acetylacetone-tetramethylammonium chloride) in an ethanol solution to extract precipitates. The extract was filtered using a filter having a pore size of 20 nm, and precipitates having a particle diameter of less than 20 nm were separated, and the precipitates were analyzed by inductively-coupled plasma optical emission spectrometry. The amount of Ti contained in the precipitate having a particle diameter of less than 20 nm was determined, and the amount of Ti contained in the precipitate having a particle diameter of less than 20 nm was divided by the amount of Ti contained in the hot-rolled steel sheet to determine a particle diameter of less than 20 nm. The proportion (percentage) of Ti contained in the precipitate.

(ii) Tensile test

For each hot-rolled steel sheet, three Japanese Industrial Standards (JIS) are used as the direction of the right angle with respect to the rolling direction. The tensile test piece No. 5 was subjected to a tensile test (strain rate: 10 mm/min) prescribed in accordance with JIS Z 2241 (2011), and tensile strength and total elongation were measured. Each of the hot-rolled steel sheets was subjected to a tensile test three times, and the average value of the three times was defined as tensile strength (TS) and total elongation (El).

(iii) Reaming test (mass production edge processing evaluation)

A test piece (size: 150 mm × 150 mm) was taken from the hot-rolled steel sheet, and a hole having an initial diameter d ₀ was formed on the test piece by press working using a 50 mmφ punch (punching gap: 30%). Then, a punch having a vertex angle of 60° was inserted into the formed hole from the punch side at the time of punching, and the hole was expanded to measure the hole diameter d ₁ when the crack penetrated the plate thickness of the steel plate (test piece). The punching ratio (%) was calculated by the following formula.

Punching rate (%)={(d ₁ -d ₀ )/d ₀ }×100

The case where the punching ratio was 30% or more was evaluated as good productivity in mass production.

The results obtained are shown in Table 3.

The hot-rolled steel sheets (No. 1 to No. 3, No. 5, No. 6, No. 9, No. 12 to No. 16, No. 21, No. 22) of the present invention are required for both. Tensile strength (900 MPa or more) and excellent mass-produced hot-rolled steel sheets. On the other hand, the hot-rolled steel sheets (No. 4, No. 7, No. 8, No. 10, No. 11, No. 17 to No. 20) of the comparative examples which are out of the scope of the present invention cannot be secured to a predetermined height. Strength does not ensure sufficient rush rate.

Claims (20)

A high-strength hot-rolled steel sheet having a composition in which S, N, Ti, and V satisfy the following formula (1), and contains, by mass%: C: 0.06% or more and 0.13% Hereinafter, Si: less than 0.5%, Mn: more than 0.5% and 1.4% or less, P: 0.05% or less, S: 0.005% or less, N: 0.01% or less, Al: 0.1% or less, and Ti: 0.05% or more and 0.25. % or less, and V: more than 0.15% and 0.4% or less, the remainder contains Fe and unavoidable impurities; and the above-mentioned high-strength hot-rolled steel sheet has a ferrite-grain phase fraction exceeding 90%, and precipitation of Ti-containing carbonization And more than 70% of the above-mentioned carbides have a particle diameter of less than 9 nm, Ti*+V≧0.35 (1) wherein, in the formula (1), Ti*=Ti-N×(48/14) -S × (48/32), S, N, Ti, and V are contents (% by mass) of each element. The high-strength hot-rolled steel sheet according to claim 1, wherein 50% by mass or more of Ti is precipitated as a precipitate containing Ti having a particle diameter of less than 20 nm. The high-strength hot-rolled steel sheet according to the first aspect of the invention, which contains, in addition to the above composition, Nb: 0.002% or more and 0.1% or less by mass%. In the high-strength hot-rolled steel sheet according to the second aspect of the invention, in addition to the above composition, Nb is contained in an amount of 0.002% or more and 0.1% or less by mass%. The high-strength hot-rolled steel sheet according to the first aspect of the invention, further comprising, in addition to the above composition, Cu: 0.005% or more and 0.2% or less, and Ni: 0.005% or more and 0.2% or less, and Cr: 0.002% or more and 0.2% or less, Mo: 0.002% or more and 0.2% or less, and Sn: 0.005% or more and 0.2% or less. The high-strength hot-rolled steel sheet according to the second aspect of the invention, in addition to the above composition, further contains, by mass%, Cu: 0.005% or more and 0.2% or less, and Ni: 0.005% or more and 0.2% or less, and Cr: 0.002% or more and 0.2% or less, Mo: 0.002% or more and 0.2% or less, and Sn: 0.005% or more and 0.2% or less. The high-strength hot-rolled steel sheet according to the third aspect of the invention, which further comprises, in addition to the above composition, Cu: 0.005% or more and 0.2% or less, and Ni: 0.005% or more and 0.2% or less, and Cr: 0.002% or more and 0.2% or less, Mo: 0.002% or more and 0.2% or less, and Sn: 0.005% or more and 0.2% or less At least one of them. The high-strength hot-rolled steel sheet according to the fourth aspect of the invention, further comprising, in addition to the above composition, Cu: 0.005% or more and 0.2% or less, and Ni: 0.005% or more and 0.2% or less, and Cr: 0.002% or more and 0.2% or less, Mo: 0.002% or more and 0.2% or less, and Sn: 0.005% or more and 0.2% or less. The high-strength hot-rolled steel sheet according to the first aspect of the invention, which contains, in addition to the above composition, B: 0.0002% or more and 0.003% or less by mass%. The high-strength hot-rolled steel sheet according to the second aspect of the invention, in addition to the above composition, further contains B: 0.0002% or more and 0.003% or less by mass%. The high-strength hot-rolled steel sheet according to the third aspect of the invention, which contains, in addition to the above composition, B: 0.0002% or more and 0.003% or less by mass%. The high-strength hot-rolled steel sheet according to the fourth aspect of the invention, in addition to the above composition, further contains B: 0.0002% or more and 0.003% or less by mass%. The high-strength hot-rolled steel sheet according to the fifth aspect of the invention, which contains, in addition to the above composition, B: 0.0002% or more and 0.003% or less by mass%. The high-strength hot-rolled steel sheet according to claim 6 of the patent application, wherein In addition to the above composition, B: 0.0002% or more and 0.003% or less are contained in mass%. The high-strength hot-rolled steel sheet according to the seventh aspect of the invention, which contains, in addition to the above composition, B: 0.0002% or more and 0.003% or less by mass%. The high-strength hot-rolled steel sheet according to claim 8, wherein, in addition to the above composition, B: 0.0002% or more and 0.003% or less are contained in mass%. The high-strength hot-rolled steel sheet according to any one of the above-mentioned items, wherein, in addition to the above composition, further contains, by mass%, Ca: 0.0002% or more and 0.005% or less, and a rare earth metal: At least one of 0.0002% or more and 0.03% or less. A method for producing a high-strength hot-rolled steel sheet, which is characterized by having the first, third, fifth, seventh, ninth, eleventh, thirteenth, and fifteenth aspects of the patent application scope The steel material of the composition of the high-strength hot-rolled steel sheet according to any one of the items of the item 17 is heated to 1100 ° C or higher, and the finishing rolling temperature is (Ar ₃ + 25 ° C) or more, and the total pressure of the final two of the finishing rolling is performed. After the hot rolling at a lower rate of 60% or less, the film is cooled at an average cooling rate of 40 ° C/s or more, and coiled at a coiling temperature of 520 ° C or more and 680 ° C or less. A method for producing a high-strength hot-rolled steel sheet, which is characterized by having the first, third, fifth, seventh, ninth, eleventh, thirteenth, and fifteenth aspects of the patent application scope The steel material of the composition of the high-strength hot-rolled steel sheet according to any one of the items of the item 17 is heated to 1100 ° C or higher, and the finishing rolling temperature is (Ar ₃ + 25 ° C) or more, and the total pressure of the final two of the finishing rolling is performed. After hot rolling at a lower rate of 60% or less, cooling is performed at an average cooling rate of 40 ° C/s or more, and coiling is performed at a coiling temperature of 500 ° C or more and 640 ° C or less, and after soaking, a soaking temperature is set. It is an annealing treatment of 760 ° C or less, and is subjected to a plating treatment immersed in a hot-dip galvanizing bath. The method for producing a high-strength hot-rolled steel sheet according to claim 19, wherein the alloying treatment is performed after the plating treatment is performed.